During photosynthesis plants generate biomass by converting inorganic carbon in the form of CO2 to carbohydrates using light energy. Accordingly, increasing the efficiency of photosynthesis is a target for improving crop productivity via conventional breeding practices and transgenic approaches.
Ribulose-1,5-bis-phosphate carboxylase/oxygenase (Rubisco) catalyzes the incorporation (fixation) of CO2 into ribulose-1,5-bisphosphate to yield two molecules of 3-phosphoglycerate. Rubisco can be present in either an active or inactive state and the conversion of the inactive state to the active state by rubisco activase is required for CO2 fixation. This process entails the carbamylation of a specific lysine residue of Rubisco. When ribulose-1,5-bisphosphate from the stroma attaches to Rubisco it causes conformational changes, allowing the enzyme to catalyse the reaction of CO2 with the newly sequestered sugar bisphosphate and starting a catalytic cycle. However, in the absence of Rubisco activase (RCA), the tight binding of ribulose-1,5-bisphosphate to the uncarbamylated Rubisco inactivates Rubisco so the rate of CO2 fixation declines. RCA uncouples ribulose-1,5-bisphosphate from decarbamylated active sites and thus promotes the access of CO2 for the carbamylation (activation) of the enzyme. That is, the activation of Rubisco enhances its catalytic capacity and thus facilitates photosynthesis.
RCA is generally present in two forms, termed alpha (long form) and beta (short form), with the latter being a truncated form of the alpha protein.
Photosynthesis is reduced when the temperature exceeds the optimum range for a particular plant and the reduction in photosynthesis at elevated temperatures is linked to Rubisco deactivation due to the inhibition of RCA.
The present invention is based on the finding that complexes comprising Oryza australiensis RCA and Rubisco are active at elevated temperatures.